Nuclear magnetic resonance (NMR) well logging tools measure the properties of nuclear spins in the formation such as the signal amplitude, longitudinal (or spin-lattice) relaxation time (T1), and transverse (or spin-spin) relaxation time (T2). Information on these NMR properties aids in the determination of basic formation properties such as permeability and porosity, as well as fluid properties such as fluid type.
Measurements of formation properties are achieved by applying a series of pulses followed by or interleaved with data acquisition. Each particular measurement configuration is called a pulse sequence. For example, transverse relaxation time, T2, is often measured with the Curr-Purcell-Meiboom-Gill (CPMG) pulse sequence or other variants, in which trains of spin echoes are generated by a series of pulses. In another example, longitudinal relaxation time, T1, may be measured by applying a series of pulse sequences with variable wait time in between to encode the longitudinal recovery in spin echo amplitudes. In another example, transverse relaxation time, T2 measurement and longitudinal relaxation time T1 measurement are combined together to obtain two-dimensional information on formation fluids. In general, any nuclear magnetic resonance measurements including but not limited to the above examples may be combined to obtain multi-dimensional information on the formation or formation fluids.
Once the nuclear magnetic resonance data are acquired, a mathematical inversion process is applied to produce the distribution of measured properties that reflects the anisotropy of formation or formation fluids. For example, T2 distribution represents the distribution of pore sizes within the formation, and the area under the T2 curve is the porosity filled with formation fluids. Interpretation of pore size distribution and logarithmic mean T2 are used to calculate petrophysical parameters such as permeability and the amount of free/bound fluid.
The pulse sequence used during logging is usually determined by the measured property. In logging oilfields, the T1 and T2 properties encountered may range from a fraction of a millisecond to several seconds. The time-separation between pulses in a pulse sequence is known as the echo spacing (TE). TE must be lower than the lowest T2 to be measured in the formation, consequently, to measure formation properties with T2 on the order of fractions of milliseconds, the pulse sequence itself must contain pulse trains with TE in the order of fractions of milliseconds or lower.
The characteristics of NMR oilfield logging equipment limit the minimum echo spacing, TE, to values that can be achieved. This in turn limits the minimum value of T2 that can be measured and can prevent more in-depth understanding of the properties of a given hydrocarbon reservoir, consequently, it is desirable to devise new mechanisms to reduce the minimum echo spacing that NMR logging tools can achieve.
The inherent nature of oilfield logging makes it so that the environmental conditions under which the logging equipment operates change over time as the device traverses through different layers of the formation. It is known that the electrical response of the logging equipment can be affected by some of those environmental conditions. This can lead to performance degradation, and often imposes the need for post-processing of the logged data to compensate for those variations. It is desirable to implement mechanisms to compensate for formation variability in an automated fashion. These mechanisms could allow for measurement compensation and performance optimization in real-time as the formation is being logged, and preclude the need for data post-processing.